noun. Marriage bestows many gifts. Some of them include diabolical, Biblical, underworld overlords.
Real citation: “This was a gift, albeit a hateful one, from my sister-in-law (a.k.a. Satan in law) who almost had a spell when her son played at our house and I allowed the kids to have fun in the sandbox that still had last summer’s sand (horrors!).”
(April 28, 2006, StuntMom, http://www.stuntmom.com/2006/04/birthday-gift.html)
Made-up citation: "Lacking a spouse, I know little of Satans-in-law. Or should that be Satan-in-laws?"